electronics Article
Label Stacking Scenarios in Hybrid Wavelength and Code-Switched GMPLS Networks Kai-Sheng Chen School of Electrical and Computer Engineering, Nanfang College of Sun Yat-Sen University, Guangzhou 510970, China;
[email protected]; Tel.: +86-188-140-940-16 Received: 18 September 2018; Accepted: 12 October 2018; Published: 14 October 2018
Abstract: Multi-protocol label switching (MPLS) is a promising solution to implement high-speed internet protocol (IP) networks by reducing the layer number. To meet the increasing demand for data traffic, optical packet switching (OPS) is integrated under IP to provide high bandwidth to end users. Generalized MPLS (GMPLS) is perfectly compatible with the routing algorithm in IP/MPLS as it supports packet-switching functions. In this paper, we investigate the label stacking scenarios in GMPLS networks. In GMPLS, label stacking is done to reduce the node complexity by appending multiple labels to a single packet. Wavelength-division multiplexing (WDM) and optical code-division multiplexing (OCDM) signals have been widely used as identifying labels. As the labels can be permutated among the wavelengths or code dimensions, the structure of a label stack can be varied. However, studies on the relationship between label stacking scenarios and network performance are limited. To investigate this issue, we propose three label stacking models: sequential code distribution; sequential wavelength distribution, and random label distribution. The simulation results show that the sequential wavelength assignment, wherein the labels are uniformly distributed among the wavelengths, exhibits the best system performance in terms of the label-error rate (LER). Keywords: wavelength-division multiplexing (WDM); generalized multi-protocol label switching (GMPLS); optical code-division multiplexing (OCDM); label stacking
1. Introduction Internet protocol (IP) is currently the most typical technology for transmitting multimedia information [1–3] with a hierarchical structure of multiple layers. These layers process the data packets in order, where the upper layer calls the lower one when completing the provided services. Due to the rapid increase in network traffic, an extended version of IP is required to support high bit-rate services for a large number of users. Multi-protocol label switching (MPLS), developed by Internet Engineering Task Force (IETF), is a promising solution to implement high-speed IP networks [4,5]. By reducing the layer number and the routing complexity, MPLS provides an efficient protocol for packet switching. The agreements in MPLS record the packet destination in the header and establish a pre-defined path between the start and the end point. During the packet propagation, the previous node identifies the label of the received packet and determines the node for setting up the next connection. Instead of analyzing the whole network, the intermediate nodes switch the packets by simply performing label recognition, which effectively saves processing time. As the complex routing algorithm is only calculated at the terminal nodes, the switching delay for an entire end-to-end route is greatly reduced. Recently, IP services have extended to multiple high-bandwidth applications, such as IP television (IPTV) [6], voice over IP (VoIP) [7], and two-way video conferences [8]. To meet the increasing demand for data traffic, optical packet switching (OPS) [9–11] is integrated under IP to provide high bandwidth and signal reliability to end users. Similar to IP structure, OPS is a node-by-node network routing the optical packets through core nodes. Before entering the optical network, the packets are converted from Electronics 2018, 7, 251; doi:10.3390/electronics7100251
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electrical to optical domain by the optical edge nodes located in the boundary between IP and OPS. As OPS supports label-based functions, such as label generation and label processing, it is perfectly compatible with the routing algorithm in IP/MPLS. Generalized multi-protocol label switching (GMPLS) is considered a future candidate for metropolitan area networks (MANs) and OPS [12–14]. By practically using the important parts of IP, GMPLS can provide a greater network capacity and flexible user traffic. The network forwards a packet to its destination by processing its labels to decide the switching path. During the switching procedure, the packet is preserved in the optical domain. The network expenses can be reduced by avoiding the costly operation of optical-to-electrical conversion in the routers. Label stacking is a smart-switching criterion in GMPLS for fast and reliable transmission [15,16]. Label stacking and optical label swapping [17,18] are the two main methods used to switch packets through multiple network routers. For label swapping, each router recognizes the labeled packet and swaps the old appending label. Based on a look-up table, a newly created label is assigned to the packet before it is forwarded to the next router. Unlike label swapping wherein one packet carriers only one label at a time, label stacking assigns a stack of multiple labels to a single packet. The label stack includes all the information of the switching path. The routers can decide the correct switching path by simply checking the stacked labels. A simple backbone structure can be obtained by avoiding the repeated processing involved in label swapping and label creation in routers. As the number of labels in the stack is limited, the router can make the switching decision using the look-up table with a reduced size. In typical GMPLS networks, wavelength division multiplexing (WDM) and optical code-division multiplexing (OCDM) schemes are employed to construct optical labels [17,18]. The label of a packet can be expressed as (W, C), where W and C denote the WDM channel and the code used for transmitting the packet data, respectively. In this approach, each WDM channel is shared by multiple OCDM labels. The packet is forwarded via an end-to-end route comprising multiple routers and connected paths between the routers. A label distribution protocol (LDP) helps decide the label corresponding to each intermediate path. For a given route, there are several combinations of assigned labels provided by the LDP. The scenarios of label distribution significantly affect the network performance in label switching systems. In Reference [17], multi-service transmission was realized by modifying the number of OCDM labels among WDM channels. In another study [18], the effects of different labeling schemes on a hybrid wavelength/code buffering structure were evaluated. To obtain the best performance, the router intelligently converts the wavelength of the label into a new one with the least occupied OCDM labels. However, only label swapping schemes have been discussed in the above studies. For GMPLS networks with label stacking, the relationship between the label distribution scenarios and the network performance is rarely studied. In this paper, we propose label stacking scenarios to provide packet switching service in hybrid wavelength and code-switched GMPLS networks. Unlike previous studies, this study focuses on the relationship between label distribution methods and network performance. As the labels can be permutated among the wavelengths or code dimensions, the structure of a label stack can be varied. We propose three representative label stacking models: sequential code distribution; sequential wavelength distribution, and random label distribution. We perform numerical simulations to compare the effectiveness of the three labeling schemes. The system performance is evaluated in terms of the label-error rate (LER) by analyzing the power spectral density (PSD) of the label distribution models. 2. Related Work Combining the advantages of IP and OPS, GMPLS enables the nodes to execute packet switching in time, wavelength, and space domain [19–23]. Multiple optical labeling schemes have been proposed as the replacements for the conventional modules of electrical header processing. The available methods of generating optical labels include on–off keying (OOK) [19], frequency-shift keying (FSK) [20], and bit
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available methods of generating optical labels include on–off keying (OOK) [19], frequency-shift keying (FSK) [20], and bit staking [21]. Except for the above scenarios, wavelength division multiplexing alsothe plays a significant in integrating MPLS over optical(WDM) layers [22,23]. In staking [21]. (WDM) Except for above scenarios,role wavelength division multiplexing also plays wavelength the labelMPLS expressed as the signal is analyzed to determine the a significantswitching, role in integrating over optical layers wavelength [22,23]. In wavelength switching, the label switching the packets. After previous label is identified, it is path swapped a newAfter one by expressed path as theofsignal wavelength is the analyzed to determine the switching of thefor packets. the changing the wavelength receivedfor signal. employing WDM labels the previous label is identified,ofit the is swapped a newAlthough one by changing the wavelength of reduces the received complexity of deploying all-optical networking supports high-speed packet switching, some signal. Although employing WDM labels reducesand the complexity of deploying all-optical networking challenges remain unsolved. For example, during the entire packet interval, only a during single and supports high-speed packet switching, some challenges remain unsolved. For example, wavelength is used throughout the label bandwidth, in the reduction of bandwidth the entire packet interval, only a single wavelength is usedresulting throughout the label bandwidth, resulting efficiency. in the reduction of bandwidth efficiency. Multi-wavelength Multi-wavelengthsignals signalsare areanother anothersolution solutionto toimplement implementoptical opticallabels labelsas asthey theyare areefficient efficient in inbandwidth bandwidthutilization utilization[24] [24]and and compatible compatiblewith withlabel label stacking stacking[25]. [25]. Such Suchlabels labelsachieve achieverelatively relatively high highswitching switchingspeed speedwhile whilecontrolling controllingthe thecost costwithin withinan anacceptable acceptablerange. range.The TheLabel Labelof ofan anoptical optical packet multiplewavelengths wavelengthsbased basedononthe the label structure, example, weight-2 labels packet consists of multiple label structure, forfor example, weight-2 labels [25]. [25]. The address information firstly encoded on the of spectrum ofsignal, the optical before The address information is firstlyisencoded on the spectrum the optical before signal, appending the appending the multi-wavelength label to a packet. multi-wavelength label to a packet. Optical Optical code code labeling labeling (OCL), (OCL), mapping mapping the the packet packet address address onto onto the the label label through through optically optically encoding, is proposed as a promising labeling method in GMPLS networks [26–28]. Optical encoding, is proposed as a promising labeling method in GMPLS networks [26–28]. Opticalcode codeisis employed employedto to switch switch data data flows flows based based on on aa multiplexing multiplexing technique technique known known as as optical optical code-division code-division multiple multipleaccess access(OCDMA) (OCDMA)[29–33]. [29–33].The Thespeed speedlimitation limitationofofprocessing processingaalabel labelcode codeisisovercome overcomeby by avoiding executing optical-to-electrical conversion at core nodes. In References [26,27], optical avoiding executing optical-to-electrical conversion at core nodes. In References [26,27], optical carries carries were modulated with orthogonal generated from an optical encoder on were modulated with orthogonal opticaloptical codes codes generated from an optical encoder basedbased on array array waveguide grating (AWG). Authors of Reference [28] had experimented recognizing the labels waveguide grating (AWG). Authors of Reference [28] had experimented recognizing the labels of of optical codesusing usingananoptical opticalcorrelator correlatormade madeup upofofpassive passive optical optical components. components. The optical codes The incoming incoming code code was was correlated correlated with with all all available available codes codes to to calculate calculate the the correlation correlation results results for for identifying identifying the the received label. received label.
3.Label LabelStacking Stackingin inHybrid HybridWavelength Wavelength and and Code-Switched Code-Switched GMPLS GMPLS Network Network 3. The hybrid hybrid wavelength wavelength and and code-switching code-switching technique technique employed employed in in GMPLS GMPLS networks networks helps helps The increase the speed and reduce the complexity of performing all-optical networking. increase thepacket packetswitching switching speed and reduce the complexity of performing all-optical In this approach, the approach, identifyingthe label (expressedlabel as optical codes and wavelength are used to networking. In this identifying (expressed as optical codessignals) and wavelength switch the through paths. We classify thepaths. fiber waveband into WDM channels signals) arepacket used to switchmultiple the packet through multiple We classify theseveral fiber waveband into and assign multiple optical each of them. shows the several WDM channels andcodes assigntomultiple opticalFigure codes1 to each the of classification them. Figure scheme 1 showsof the hybrid code/wavelength labels. In code/wavelength this figure, M denotes theInnumber of channels, andthe N denotes classification scheme of the hybrid labels. this figure, M denotes number the of number ofand codes a channel. Accordingly, the total of available labels (L) be channels, N multiplexed denotes theinnumber of codes multiplexed innumber a channel. Accordingly, thecan total obtainedofasavailable L = M ×labels N. (L) can be obtained as L = M × N. number
Figure 1. Classification of the wavelength-division multiplexing (WDM)/ optical code-division Figure 1. Classification of the wavelength-division multiplexing (WDM)/ optical code-division multiplexing (OCDM) labels. multiplexing (OCDM) labels.
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The switching mechanisms in the proposed GMPLS network are supported by the label stacking technique. In such a network, the LDP maps each connection between the nodes to a The switching mechanisms in the proposed GMPLS network are supported by the label stacking specific label. At the network input, an edge router (ER) is used to determine the label switched path technique. In such a network, the LDP maps each connection between the nodes to a specific label. (LSP)Atofthethe transmitted packets based(ER) on istheir Alllabel the switched labels corresponding to the network input, an edge router useddestinations. to determine the path (LSP) of the connections in the LSP are included in a label stack. The ER labels the packets with the stack transmitted packets based on their destinations. All the labels corresponding to the connections in and sendsthethe to thein next router (CR). Totheperform packet CRsthe recognize LSPpackets are included a labelcore stack. The ER labels packets with the switching, stack and sends packets the received stackrouter by identifying its codes wavelengths. The outgoing path is stack determined to thelabel next core (CR). To perform packetand switching, CRs recognize the received label by its codesand and de-multiplexing wavelengths. The results outgoing is determined basedidentifying on the decoding ofpath the stacked labels.based on the decoding and de-multiplexing of the stacked scheme labels. in an exemplary GMPLS network with two LSPs. In Figure 2 showsresults the label stacking Figure 2 shows the label stacking scheme an (W exemplary GMPLS network with two LSPs. LSP1, the packets are labeled as (W1, C1), (W 1, C2),in and 3, C1). CR1 switches the packets along LSP1 In LSP1, the packets are labeled as (W , C ), (W , C ), and (W , C ). CR1 switches the packets along 1 method 2 proposed 1 Reference [30]. Similarly, CR2 and 1 the 1 3 in by identifying (W1, C1) in the stack using LSP1 by identifying (W 1 , C1 ) in the stack using the method proposed in Reference [30]. Similarly, CR3 forward the packets by realizing the labels (W1, C2) and (W3, C1), respectively. In this network, CR2 and CR3 forward the packets by realizing the labels (W 1 , C2 ) and (W 3 , C1 ), respectively. In this each CR only processes label recognition, and the packets are switched without swapping their network, each CR only processes label recognition, and the packets are switched without swapping labels. their labels.
Figure 2. Label stacking hybridwavelength wavelength and and code-switched multi-protocol labellabel Figure 2. Label stacking in in hybrid code-switchedgeneralized generalized multi-protocol switching (GMPLS) network. switching (GMPLS) network.
4. Methods of Stacking Hybrid Wavelength/Code Labels
4. Methods of Stacking Hybrid Wavelength/Code Labels 4.1. Sequential Code Distribution
4.1. Sequential Distribution In thisCode approach, the labels are sequentially assigned to the connections in an LSP based on the code index i, where 1 ≤ i ≤ N. When (W 1 , C1 ) is assigned to a connection, the label (W 2 , C1 ) In this approach, the labels are sequentially assigned to the connections in an LSP based on the allocated to the same WDM channel is selected for the next connection. When all N labels in the code index i, where 1 ≤ i ≤ N. When (W1, C1) is assigned to a connection, the label (W2, C1) allocated first WDM channel W 1 are occupied, the labels in the second WDM channel, i.e., (W 2 , C1 ), (W 2 , C1 ) to the same WDM channel is selected for the next connection. When all N labels in the first WDM . . . , are sequentially assigned to the remaining connections, as shown in Figure 3a. In this scenario, channel 1 are occupied, the labels in the second WDM channel, i.e., (W2, C1), (W2, C1)…, are mostW labels are stacked in a few WDM channels. Figure 3b shows the PSD of the stacked labels of the sequentially assigned to the remaining connections, as shown Figure 3a. In code this in scenario, sequential code distribution. When a new label is assigned to the in stack, an unused the samemost labelsWDM are stacked a few WDM channels. Figure shows labels the PSD of the stacked of the channel isinselected. In this example, most of the3b occupied are distributed in the labels channels sequential code When a new label is assigned to the stack, an unused code in the same of W 1 and Wdistribution. 2.
WDM channel is selected. In this example, most of the occupied labels are distributed in the channels of W1 and W2.
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(W1, C2) (W1, C2) Power Power (W1, CN) (W2, CN) (W (W11,,CCN-1 (W22,,CCN-1 N)) (W N)) (W1, CN-1) (W2, CN-1)
(W1, C2) (W1, C12) (W1, C1)
(W2, C2) (W2, C12) (W2, C1)
(W2, C1) (W2, C1) (a) (a)
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... ... (W2, CN) (W2, CN)
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(W3, C1) l (W3, C1) (b) l (b) Figure 3. Sequential code distribution: (a) label assignment to the label switched path (LSP), (b) Figure 3. Sequential code distribution: (a) label assignment to the label switched path (LSP), (b) power Figure spectral 3. Sequential distribution: (a) labels. label assignment to the label switched path (LSP), (b) power densitycode (PSD) of the stacked spectral density (PSD) of the stacked labels. power spectral density (PSD) of the stacked labels.
4.2. 4.2.Sequential SequentialWavelength WavelengthDistribution Distribution 4.2. Sequential Wavelength Distribution In Inthis thisscenario, scenario,when when(W (Wi ,i,CCj )j)isisoccupied occupiedby byaanode, node,the thelabel label(W (W i+1,,C Cj )j)isissequentially sequentiallychosen chosen i+1 In this scenario, when (W i , C j ) is occupied by a node, the label (W i+1 , C j ) is sequentially for where 11≤≤i i≤≤MM− 1−and 1 and j ≤Figure N. Figure 4a shows the mapping of the forthe the next next connection, where 1 ≤1j ≤ N. 4a shows the mapping of thechosen labels forthe theto next connection, where 1the ≤for istructure ≤ the M −structure 1 and 1 ≤label N. Figure 4a shows the mapping the labels labels theconnections. path connections. ofj ≤ the label stack, for example, when , C to path As forAs of the stack, for example, when (W 1, Cof 1(W ) is chosen, 1 ) is 1 to the path connections. As for the structure of the label stack, for example, when (W 1 , C 1 ) is chosen, chosen, , C1 ) is assigned to the stack. Similarly, this procedure is repeated C11)…(W ), (W 4M, ,CC11)) (W2, C1)(W is 2assigned to the stack. Similarly, this procedure is repeated for (W3, Cfor 1), (W 3 4,, C , Call 1), is thechannels stack. Similarly, thisSubsequently, procedure repeated 3, 2C 1), ), 4assigned ,C )…(W M C1) . (W . . 2(W C1assigned ) channels until alltothe have exactly one label.isSubsequently, C (W . ,are until the have exactly one label. (W1, C2),for (W(W 2(W ,C the 2 (W 2 ) . .to 1 ,)…are 2 ,1C M until all the channels exactly Subsequently, (W1, C2the ), (W 2,similar C2)…are assigned to the assigned to nodes the remaining nodes in aone proper In this channels have similar label remaining in a have proper order. In label. this order. scenario, the scenario, channels have label numbers, as remaining nodes in a proper order. In this scenario, the channels have similar label numbers, as numbers, shown inWhen Figurea4b. When new label isthe stacked, thethe onesame with code the same code index as the shown inas Figure 4b. new labela is stacked, one with index as the previous shown Figure 4b. When a new label is stacked, the one with the same code index as the previous previous label is selected. label is in selected. label is selected.
Figure 4. Sequential wavelength distribution: (a) label assignment to LSP, (b) PSD of the stacked labels. Figure 4. Sequential wavelength distribution: (a) label assignment to LSP, (b) PSD of the stacked Figure 4. Sequential wavelength labels. Performance 5. Network Analysis distribution: (a) label assignment to LSP, (b) PSD of the stacked labels.
In this section, we perform numerical simulations to evaluate the system performance in terms 5. Network Performance Analysis of the LER. To detect a specific hybrid WDM/OCDM label from the stack, both de-multiplexing 5. Network Performance Analysis In this section, we perform numerical simulations to evaluate the can system performance terms and decoding processes are required. A nearly ideal de-multiplexing be performed byin simply InLER. this To section, we performhybrid numerical simulations to evaluate the system performance in terms of the detect a specific WDM/OCDM label from the stack, both de-multiplexing using an optical band-pass filter. As for the decoding, multiple access interference (MAI) shouldand be of the LER. To detectare a specific hybrid WDM/OCDM label from the stack, both de-multiplexing and decoding processes required. A nearly de-multiplexing be WDM performed by simply using considered, as the target label and the othersideal are multiplexed in thecan same channel. The OCDM decoding processes arefilter. required. A nearly ideal de-multiplexing can interference be performed(MAI) by simply using an opticalwith band-pass As for the (SAC) decoding, access should be technique spectral amplitude coding [31] ismultiple employed, as it can eliminate MAI. The desired an optical band-pass filter. decoding, multiple inaccess interference (MAI)The should be considered, as the target labelAs andfor thethe others are multiplexed the same WDM channel. OCDM considered, as the target label and the others are multiplexed in the same WDM channel. The OCDM
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technique with spectral Electronics 2018, 7, 251
amplitude coding (SAC) [31] is employed, as it can eliminate MAI.6 of The 11 desired SAC label can be recovered without the influence of MAI using designated signature codes and balanced detection [32]. The factors affecting the performance of SAC systems are noise sources SAC can benoise, recovered the phase-intensity-induced influence of MAI using noise designated codes such label as thermal shot without noise, and (PIIN).signature Therefore, we and can balanced detection [32]. The factors affecting the performance of SAC systems are noise sources such define the LER of the SAC labels as follows: as thermal noise, shot noise, and phase-intensity-induced noise (PIIN). Therefore, we can define the LER of the SAC labels as follows: I P2 1 LER erfc (1) 2 2 2 2 s 2 iTH 2iPIIN iS ! IP 1
2 2 2 LER = erfc (1) 2 2 iTH + iPIIN + iS where erfc(·) denotes the complementary error function, Ip denotes the photocurrent in Ampere,
2 2 2 denotes the variance of thermal noise defined in [32], denotes the variance of PIIN,iTH and iTH iPIIN where erfc(·) denotes the complementary error function, Ip denotes the photocurrent in Ampere,
2
2 2 denotes the variance of thermal noise defined in [32], toiPIIN denotes the variance of PIIN, and iS the variance of shot noise. According the definition given in Reference [32], the iS denotes denotes the variance of shot noise. According to the definition given in Reference [32], the received
2obtain
22 and i 2 . received PSD of thelabels stacked labels at themust decoder must to beobtain given to PIIN PSD of the stacked at the decoder be given iTH and iiTH PIIN . we assume assume that the PSD of the un-encoded light source is flat and un-polarized, For simplicity, we wavelength of of the the first first WDM WDMchannel channelisisshifted shiftedto toλλc c== v/2M, v/2M, where v is the width of and the central wavelength the light light spectrum. spectrum. Assuming Assumingthe thejth jthlabel labelcode codevector vector C j = [c j (1), c j (2)…c Cj = [cj (1), cj (2) . . . cjj(N)], (N)], where where cjj(h) (h) is the hth (W 1, 1 C, 1C ) is as as shown in Figure 5. In chip of Cjj and and 11 ≤≤hh≤≤N,N,the thePSD PSDofofa asingle singleSAC SAClabel label (W is obtained, shown in Figure 5. 1 ) obtained, thisthis figure, thethe optical spectrum In figure, optical spectrumwith withbandwidth bandwidthv visisoccupied occupiedby by M M WDM WDM channels, channels, and each channel is is sliced slicedinto intoNNwavebands wavebandswith witha awidth widthofofv/MN. v/MN.The Thewavebands wavebands with a pulse correspond channel with a pulse correspond to to the chip “1s” inj ,Cwhereas j, whereas chip “0s” corresponds to the bands with zero power. the chip “1s” in C thethe chip “0s” corresponds to the bands with zero power.
Figure 5. PSD of a single spectral amplitude coding (SAC) label (W 1 , C1 ). Figure 5. PSD of a single spectral amplitude coding (SAC) label (W1, C1).
In the following analysis, we assume that (W 1 , C1 ) is the target label to be identified. Therefore, In the following analysis, we assume that (W1, C1) is the target label to be identified. Therefore, the decoder only processes the stacked labels in W 1 , and the signals allocated to the other WDM the decoder only processes the stacked labels in W1, and the signals allocated to the other WDM channels are filtered out. Based on Figure 5, the PSD of the stacked SAC labels in W 1 can be channels are filtered out. Based on Figure 5, the PSD of the stacked SAC labels in W1 can be mathematically expressed as follows: mathematically expressed as follows: P KP1 K1 N N λ − vv h − 1 1 G (λ) = c h Π (2) ( ) ∑ cjj h l MN h 2 G (lMv ) ∑ (2) MN 2 jMv = 1 jh1=h 11 where PPisisthe thereceived receivedoptical optical power, K1the is stacked the stacked number W1.symbol The symbol where power, andand K1 is labellabel number in W 1in . The Π(λ) the rectangle function centered λ and with a width v/MN.For ForSAC SAClabels labels using using (l ) represents represents the rectangle function centered at λatand with a width of of v/MN. photocurrent IIpp of the decoded label (W11,, C M-sequence codes [33], the photocurrent C11))can canbe beobtained obtainedas asfollows: follows: v/M N RP N 1 v 1 I p RGN(l ) c1 h c1 h l v h 1 dl (3) RP( N + 1) Ip = RG (0 λ) ∑ [hc11 (h) − c1 (h)]Π λ − MN h − 2 dλ = 2MN (3) MN 2 2MN h=1 0 where R is the responsivity of the photo-diode and c1 h is the two’s complement of c1(h). The where R is of thethe responsivity thePIIN photo-diode and c1 (h)as is the two’s complement of c1 (h). The variances variances shot noiseof and can be expressed follows: of the shot noise and PIIN can be expressedv / MN as follows: eBRPK1 N 1 iS2 2eB G l d l (4) v/MN MN Z 0 D E eBRPK N + 1 ( ) 1 iS2 = 2eB G (λ)dλ = (4) MN
v/M Z
0
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D
2 iPIIN
E
= BR
2
v/MN Z
G2 (λ)dλ =
0
R2 P2 BK1 ( N + 1)(K1 + 1) 4MNv
(5)
where B is the electrical bandwidth of the decoder, and e is the electron charge in Coulomb. However, for a network system with K stacked labels, the labels in W 1 for the two distribution scenarios have different quantities, which can be described as follows: ( K, K ≤ N , for sequential code distribution K, K > N K1 = j k K +1 , for sequential wavelength distribution M
(6)
where b·c denotes the floor function. Furthermore, we introduce the third scenario called random label distribution. In this scenario, except for (W 1 , C1 ), the other K − 1 labels are randomly selected from the M × N labels in the label domain. The expected label number K1 for random label distribution in the first WDM channel W 1 can be expressed as follows:
K1 =
( M −1) N K C N −1 C 1 + ∑ (h − 1) h−C1 MNK−−1h , h=1 N
1 + ∑ ( h − 1) h=1
K −1 ( M −1) N ChN−−11 CK −h MN −1 CK −1
,
K≤N (7) N < K < ( M − 1) N
6. Simulation Results and Discussion Figure 6 compares the LERs of the three label stacking scenarios in the GMPLS network for hybrid labels; here, (M, N) = (8, 31) and (M, N) = (4, 63). The parameters used for the simulations are the received power (P = −10 dbm), photo-diode responsivity (R = 0.81 A/W), and electrical bandwidth (B = 80 MHz). The LER of the sequential wavelength distribution is better than those of the sequential code distribution and random label distribution. This is because distributing all the labels among the wavelengths helps reduce the number of overlapping labels in the WDM channel, resulting in a lower noise variance. The system performance reduces when K1 is high, where PIIN variance is high. Given a fixed K, M = 4 is better than the case of M = 2, as the label number in channel W 1 is lower. In addition, this figure shows the LER of the standard SAC labels, which can be expressed as the hybrid label in a single WDM channel (M = 1, N = 255). The SAC outperforms the sequential code distribution as the label stack is encoded in the entire bandwidth rather than the narrow WDM channels. However, its performance still cannot compete with the proposed scenario of sequential wavelength distribution. Figure 7 shows the effect of the received optical power on the system LER for K = 80 and (M, N) = (8, 31). The LERs of the random label distribution and sequential wavelength distribution decrease with the increase in the received power. In the two cases, the stacked label numbers are relatively low, and the main factors affecting the LER are the thermal and shot noises. The two noise sources can be mitigated by increasing the intensity of the photocurrent. However, this tendency is not evident for the sequential code distribution and SAC labels, wherein the PIIN effect is considerable because of the high stacked label number, K1 = 31 and 80, respectively. As the variance of PIIN is proportional to the signal power, the LER cannot be reduced by simply employing a high-intensity light source. Figure 8 shows the LERs of the three stacking scenarios with respect to the number of WDM channels. The label number is 120. The stacking schemes with a higher M are found to exhibit better LERs. This is because of the fewer labels stacked in the WDM channels. Moreover, the sequential wavelength assignment once again outperforms the other two in terms of the LER, which is consistent with the results shown in Figure 6. The performance of the stacked SAC labels is not investigated in this figure, as the channel number M is always fixed to 1.
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Figure 6. Label-error rate (LER) versus K1 for the three label stacking scenarios. C: sequential code assignment; W: sequential wavelength assignment; R: random label assignment.
Figure 7 shows the effect of the received optical power on the system LER for K = 80 and (M, N) = (8, 31). The LERs of the random label distribution and sequential wavelength distribution decrease with the increase in the received power. In the two cases, the stacked label numbers are relatively low, and the main factors affecting the LER are the thermal and shot noises. The two noise sources can be mitigated by increasing the intensity of the photocurrent. However, this tendency is not evident for the sequential code distribution and SAC labels, wherein the PIIN effect is considerable because of the high stacked label number, K1 = 31 and 80, respectively. As the variance of PIIN is proportional to the signal power, the LER cannot be reduced by simply employing a high-intensity light source. Figure 6. Label-error rate (LER) versus K1 for the three label stacking scenarios. C: sequential code Figure 6. Label-error rate (LER) versus K1 for the three label stacking scenarios. C: sequential code assignment; W: sequential wavelength assignment; R: random label assignment. assignment; W: sequential wavelength assignment; R: random label assignment.
Figure 7 shows the effect of the received optical power on the system LER for K = 80 and (M, N) = (8, 31). The LERs of the random label distribution and sequential wavelength distribution decrease with the increase in the received power. In the two cases, the stacked label numbers are relatively low, and the main factors affecting the LER are the thermal and shot noises. The two noise sources can be mitigated by increasing the intensity of the photocurrent. However, this tendency is not evident for the sequential code distribution and SAC labels, wherein the PIIN effect is considerable because of the high stacked label number, K1 = 31 and 80, respectively. As the variance of PIIN is proportional to the signal power, the LER cannot be reduced by simply employing a high-intensity light source.
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Figure 7. LER versus P for the three label stacking scenarios.
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Figure 7. LER versus P for the three label stacking scenarios.
Figure 8 shows the LERs of the three stacking scenarios with respect to the number of WDM channels. The label number is 120. The stacking schemes with a higher M are found to exhibit better LERs. This is because of the fewer labels stacked in the WDM channels. Moreover, the sequential wavelength assignment once again outperforms the other two in terms of the LER, which is consistent with the results shown in Figure 6. The performance of the stacked SAC labels is not investigated in this figure, as the channel number M is always fixed to 1. Figure 7. LER versus P for the three label stacking scenarios.
Figure 8 shows the LERs of the three stacking scenarios with respect to the number of WDM channels. The label number is 120. The stacking schemes with a higher M are found to exhibit better LERs. This is because of the fewer labels stacked in the WDM channels. Moreover, the sequential wavelength assignment once again outperforms the other two in terms of the LER, which is consistent with the results shown in Figure 6. The performance of the stacked SAC labels is not investigated in this figure, as the channel number M is always fixed to 1. Figure 8. LER versus WDM channel number M. for the three label stacking scenarios.
Figure 8. LER versus WDM channel number M. for the three label stacking scenarios.
7. Conclusions and Future Works In this paper, we investigated the label stacking scenarios in a hybrid wavelength and code-switched GMPLS network. The relationship between the label stacking structure and the
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7. Conclusions and Future Works In this paper, we investigated the label stacking scenarios in a hybrid wavelength and code-switched GMPLS network. The relationship between the label stacking structure and the network performance was studied in terms of the LER. The simulation results showed that the LER performance of the sequential wavelength distribution is better than those of the random label distribution and sequential code distribution. Equivalently spreading the labels over the WDM channels in the optical spectrum helps reduce the label number in a single channel, resulting in a lower noise variance. Therefore, better LER results could be achieved by assigning each channel with similar label numbers. Finally, the effects of the label power and channel number on the network LER performance were analyzed. Results reveal the stacking schemes with a large channel number are found to exhibit better LERs. The LER of sequential wavelength distribution can be further improved by employing a high-intensity light source. Evaluating the spectral efficiency in the proposed scenarios of label distribution is an exciting topic for future works. Based on the conclusion, the sequential wavelength distribution always has the best performance, but its label stack occupies a larger bandwidth when K ≤ N. Analyzing the tradeoff between LER and spectral efficiency would give an adequate depiction of the proposed network. In addition, instead of M-sequence codes used in the paper, some of SAC codes with beneficial characteristics have been employed in conventional OCDMA networks to reach small error probabilities. Exploiting other codes for constructing hybrid labels is another critical subject for further improving the proposed idea. Funding: This research received no external funding. Conflicts of Interest: The author declares no conflict of interest.
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